US 7148714 B2
A low-current pogo probe card for measuring currents down to the femtoamp region includes a laminate board having a layer of conductive traces interposed between two dielectric layers. A plurality of probing devices, such as ceramic blades, are edge-mounted about a central opening so that the probing needles or needles included therein terminate below the opening in a pattern suitable for probing a test subject workpiece. A plurality of pogo pin receiving pad sets, each including a guard pad, occupy the periphery of the board. Each guard pad is electrically connected to a trace from the layer of conductive traces. The pad sets may be connected to the probing devices by low noise cables or traces. Air trenches separate the pad sets for reducing cross talk and signal settling times.
1. A probe card comprising:
(a) a first conductive surface on said probe card;
(b) a second conductive surface on said probe card substantially surrounding said first conductive surface;
(c) a third conductive surface on said probe card;
(d) a fourth conductive surface on said probe card substantially surrounding said third conductive surface;
(e) said first, second, third, and fourth conductive surfaces are electrically insulated from one another;
(f) a first probing element supported by said probe card;
(g) a second probing element supported by said probe card;
(h) said first conductive surface electrically interconnected to said first probing element;
(i) said third conductive surface electrically interconnected to said second probing element;
(j) a first groove substantially surrounding said first conductive surface and not surrounding said third conductive surface;
(k) a second groove substantially surrounding said third conductive surface and not surrounding said first conductive surface.
2. The probe card of
3. The probe card of
This application is a continuation of and claims priority to U.S. patent application Ser. No. 11/029,745 filed Jan. 5, 2005 now U.S. Pat. No.7,068,057; which is a continuation of U.S. patent application Ser. No. 10/948,717 filed Sep. 22, 2004 now U.S. Pat. No. 7,042,241; which is a continuation of U.S. patent application Ser. No. 10/616,164 filed Jul. 7, 2003, now U.S. Pat. No. 6,856,153; which is a continuation of U.S. patent application Ser. No. 10/313,746 filed Dec. 6, 2002, now U.S. Pat. No. 6,822,467; which is a continuation of U.S. patent application Ser. No. 09/490,264 filed Jan. 24, 2000, now U.S. Pat. No. 6,559,668; which is a continuation of U.S. patent application Ser. No. 08/871,609 filed Jun. 10, 1997, now U.S. Pat. No. 6,034,533.
The present invention relates to pogo probe cards used for probing test subject workpieces, such as wafers of integrated circuit dies (“IC wafer dies”), and in particular relates to pogo probe cards suitable for use in measuring current as low as the femtoamp order of magnitude (ultra-low current).
Typically, a pogo probe card includes a dielectric board having top and bottom major surfaces and forming a base for other elements. A plurality of probing devices are mounted in radial arrangement about the top rim of a round central opening in the board. A probing needle for each of these devices terminates below the opening in a pattern suitable for probing contact sites, otherwise referred to as test sites, of an IC wafer die. For ease of description in this application the portion of the pogo probe card which in operation is closest to the IC wafer is denoted as the bottom of the card, although other geometries of test are possible.
Around the exterior periphery of the pogo probe card there are typically 48, 64 or 96 pogo pin receptive pads or pad sets, each of which is electrically connected to a respective one of the probing devices by a signal trace or set of traces. During testing, a mating pogo test head with a matching set of 48, 64 or 96 pogo pins or pin sets, touches the pogo probe card so that the pogo pins make electrical contact with the receptive pads. In this manner the probing devices are individually connected to respective channels of a test instrument by the pogo pin sets and further cabling.
In one conventional type of setup for testing IC wafer dies, the pogo probe card is mounted by a supporting rig above the IC wafer, and a chuck supports and moves the IC wafer so that each die, or region to be tested, is consecutively brought into contact with the probing needles.
As integrated circuitry has been made smaller, a need has developed for test devices which can measure ultra-low current. The typical use for this type of device is to measure IC leakage currents. These are currents that flow away from the intended current path within the IC, typically due to design flaws or fabrication artifacts.
Low-current measurements are typically performed with two conductive paths (“force” and “sense”) either reaching the test site independently (“true Kelvin” connection) or joining together in the proximity of the test site (“quasi Kelvin” connection) to form a “signal path.” The force path, whose test equipment terminus has a relatively low impedance, is provided with a particular current. The sense path, whose test equipment terminus has a very high impedance, measures the voltage at the test site. As such, the current versus voltage characteristics of the test device can be obtained using the force and sense paths.
This test configuration is desirable because although small variations in current are being measured, the amount of current directed to the test site might be large enough so that there is a significant voltage drop through the signal line leading to the test site. Because this signal line typically includes solder connections and pogo pin contacts, its resistance is impossible or impractical to predict using current technology. Therefore, the distance from the test site to the point at which the signal path splits into force and sense path is a determinant of test quality, referred to in the low-current test industry as the degree to which the test configuration approaches the ideal “true kelvin” configuration in which the force and sense paths are connected by the conductive test site itself.
Collectively, the force and sense paths are referred to as the signal path(s). On pogo probe cards the force and sense paths are typically in the form of conductive traces, both of which are on the top surface of the card.
Designers of ultra-low current probe cards have been concerned with reducing probe card leakage currents. These are unwanted currents that can flow into a first force or sense path from nearby conductive path sets, thereby distorting the current measured in the first force or sense path. The amount of leakage current between two conductive path sets is dependant on the resistivity of the insulating material that separates the paths. When measuring in the femtoamp order of magnitude, even materials which are generally thought of as being completely insulative, such as rubber or glass-epoxy, may permit a detrimental flow of leakage current.
To protect a test station from electromagnetic interference, elaborate shielding has been developed. U.S. Pat. No. 5,345,170, which is assigned to the same assignee as the present application, describes one such design.
One technique that has been used for suppressing interchannel leakage currents on pogo probe cards is providing “guard” traces on both sides of a force or sense trace on the top surface of the card which is maintained at the same potential as the signal trace on the top surface of the card by a feedback circuit in the output channel of the test instrument. Because the voltage potentials of the outer guard traces on both sides of a force or sense trace on the top surface of the card and the inner signal trace are made to substantially track each other, negligible leakage current will flow across the dielectric material of the card that separates these traces.
Although leakage current can still flow between neighboring guard traces, this is typically not a problem because these guard conductors, unlike the inner signal trace, are at low impedance. By using this guarding technique, significant improvement may be realized in the low-level current measuring capability of pogo probe cards.
Low current pogo probe cards that have force, sense, and guard traces have a “pad set” for each signal channel consisting of a force, sense, and guard pad and a corresponding “trace set” consisting of a guard trace and a combined force and sense trace.
To further improve low-current measurement capability, pogo probe cards have been constructed so as to minimize leakage currents between the individual probing devices that mount the probing needles or other needles. In these devices, higher-resistance ceramic insulating materials have been substituted for lower-resistance materials and additional guard channel conductive surfaces have been added.
In one type of assembly, for example, each probing device is constructed using a thin “blade” of ceramic material, which is a material known to have a relatively high volume resistivity. An elongate conductive trace is provided on one side of the “blade” to form the signal line and a backplane conductive surface is provided on the other side of the “blade” for guarding purposes.
The probing element of this device is formed by a slender conductive needle, such as of tungsten, which extends in a cantilevered manner away from the signal trace. Such devices are commercially available, for example, from Cerprobe Corporation based in Tempe, Ariz. During assembly of the probe card, the ceramic blades are edge-mounted in radial arrangement about the opening in the card so that the needles terminate below the opening in a pattern suitable for probing the test device.
The conductive backplane on each blade is connected to the guard trace of the corresponding pogo pin set and also to a corresponding conductive pad or “land” adjacent the central opening in the probe card. In this manner each conductive signal line is guarded by the conductive backplane on the opposite side of the blade and by the conductive land beneath it.
It has been found, however, that even with the use of guard traces and ceramic probing devices, the level of undesired leakage current is still not reduced sufficiently to match the femtoamp range measurement capabilities of the latest generation of commercially available test instruments. Thus, it has become evident that other changes in pogo probe card design are needed in order to keep pace with the technology of test instrument design.
In an additional design for a pogo probe card, each guard trace is connected by a series of plated vias through the probe card to an auxiliary guard trace that is formed on the bottom exterior surface of the card. The auxiliary guard traces parallel respective signal traces toward the center of the card.
This design leaves a number of problems still unresolved. First, leakage current may flow between the plated vias through the dielectric material that separates each guard trace from the corresponding auxiliary guard trace.
Second, the capacitance between traces of neighboring trace sets permits cross-talk. This means that a change in electric potential in a first trace set will cause a temporary change in potential in the trace of neighboring trace sets, potentially corrupting a test result.
Third, the dielectric absorption of the probe card material adjacent or interposed between neighboring trace sets results in a delay in charging and discharging the trace sets to a predetermined potential. In essence, the dielectric absorption forms a capacitor that must be charged or discharged to reach a different voltage. A test sequence for a particular IC wafer may include hundreds of brief tests. A delay of 1 second or a fraction thereof in the performance of each test, may substantially increase the total test time. By reducing the settling time of the trace sets, it may be possible to run the same number of test sequences in less time and with fewer test stations.
An additional problem encountered in prior art probe cards is the problem of probing needle damage. Probing needles occasionally break, or are otherwise damaged during testing, requiring replacement or repair. In currently available probe cards, when a probing needle breaks, the entire probing device must be replaced which is time consuming and expensive.
What is desired, therefore, is a pogo probe card with increased isolation between traces resulting in reduced leakage currents, reduced cross-talk between trace sets, and reduced settling time for each trace set.
The present invention overcomes the aforementioned drawbacks of the prior art by providing in a first aspect a pogo probe card that has auxiliary guard traces interposed between top and bottom dielectric layer of the card. Preferably, a set of conductors formed through the top dielectric layer electrically interconnects each guard trace on the top surface of the card to the auxiliary guard trace. By interposing the auxiliary guard traces between the top and bottom dielectric layers it is possible to place the auxiliary guard traces in close proximity to the guard trace on the top surface of the card, thereby reducing leakage current by reducing the cross-section for the leakage current path, but nevertheless providing a card with sufficient structural integrity.
In another aspect of the present invention, the pogo pin receptive pad sets of the pogo probe card are connected to the probe devices by coaxial or triconductor (“triax”) cables, rather than by traces on the card. This permits far greater isolation and complete guarding of each signal channel.
In a further aspect of the present invention, the probe card includes probing device holders that permit probing devices to be easily replaced when broken or damaged.
The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.
Each probing device 154 includes a probing needle 164. Probing needles 164 are typically composed of tungsten and extend generally radially inwardly in a cantilevered manner into a circular opening 166, which is defined by inner rim 156. Each probing needle 164 and probing device 154 are carefully adjusted by a technician so that the point of the probing needle 164 will contact one out of a set of test contact sites on a predetermined test subject workpiece (not shown), which is to be placed beneath card 152 for testing.
Near the outer rim of annular laminate board 158 are a plurality of pogo pin receptive pad sets 170, each of which includes a conductive force line pad 172, a conductive sense line pad 174 and a conductive guard line pad/trace 176 that surrounds both sense line pad 174 and force line pad 172.
A set of coaxial cables 180 (only one shown in
A trench 210 separates pad sets 170 from one another. Trench 210 reduces the capacitance between pad sets 170, thereby reducing cross-talk between pad sets 170 and further reducing the settling time for each individual pad set 170. The reduction in capacitance is primarily due to the much smaller dielectric constant of air compared with any of the available solid dielectric materials, such as polyimide, which has a dielectric constant approximately eight times that of air. Trench 210, which is not plated preferably extends through and therefore separates auxiliary guard traces 204 from one another. Typically in manufacture a solid conductive plate is interposed between dielectric layers 200 and 202. This layer is then divided into auxiliary traces 204 when trenches 210 are machined therein. Alternatively, even without the trench 210, the reduced cross section between the pad sets 170 and the auxiliary guard traces 204 reduces leakage currents. In this variant, the auxiliary guard traces 204 would be formed as separated metalized areas under each pad set 170.
Each blade 236 has a first and a second major surface, which are parallel to each other. On the first surface of each blade is an elongate conductive path 238 that connects probing needle 164 to an electrical connection point 239. The second major surface bears a conductive plate 240. Blade 236 is generally L-shaped in profile and is edge-mounted on the top of inner rim 156 of annular laminate board 158 so that its short arm extends through opening 166 thereby permitting needles 164 to terminate below opening 166. Blades 236 having a construction of the type just described are commercially available from Cerprobe Corporation of Tempe, Ariz.
A plurality of conductive lands 244 are formed on the laminate board 158 about the opening 166 in circumferentially spaced relationship to each other. A solder connection 248 electrically connects conductive plate 240 to a conductive land 244.
Moreover, buffer layer 254 should have sufficient conductive properties to dissipate any charge imbalances that may arise due to free electrons rubbing off the outer conductor. A suitable material for this purpose is a fluoropolymer such as TEFLON™ or other insulative material such as polyvinylchloride or polyethylene in combination with graphite or other sufficiently conductive additive.
In the field of radio frequency (rf) cable technology, cables that include a layer of the type just described are generally referred to as “low-noise” cables. Commercial sources for this type of cable include Belden Wire and Cable Company based in Richmond, Ind. and Suhner HF-Kabel based in Herisau, Switzerland. With regard to the preferred embodiment depicted, the cable which was used was purchased from Times Microwave Systems based in Wallingford, Conn.
It should be noted that some care must be exercised while connecting cable 180 to probing device 154 in order to prevent defects that would substantially degrade the low-current measuring capability probe card Referring to
Before making this connection, it is desirable to position cable 180 so that the conductive and dielectric layers in cable 180 that surround inner core 250 are set back a certain distance 262 away from the proximal edge of the probing device 154. This reduces the possibility that a fine strand of hair or other contaminant will form a low-resistance or conductive bridge so as to cause a low-resistance shunt or short across the signal line. Also, in making this connection, it is important not to overheat the cable so as not to impair the structural properties of inner dielectric 252, which material may comprise, for example, air-expanded TEFLON™ for maximum temperature stability.
Finally, after the connection has been made, all solder flux residue that remains should be removed from the board in order to prevent undesired electro-chemical effects and to maintain the surface resistivity of the laminate board 158 at a reasonable level.
In order to further reduce the possibility of undesirable shunting connections, outer conductor 256 (typically a metallic braid) of cable 180 is connected indirectly to conductive surface 240 through conductive land 244.
Moreover, with respect to probing devices 154, each elongate conductive path 238 is guarded by conductive path plate 240 on the opposite side of the blade 236 and by the corresponding conductive land 244 which is arranged below the path. Solder connection 264 electrically connects the outer conductor 256 to conductive land 244 and a second solder connection 248 electrically connects conductive land 244 to the backplane conductive surface 240. Again, care must be taken not to overheat cable 180 or to leave solder flux residue on laminate board 158.
During use of probe card 152, the signal variation or voltage is transmitted to the test site by means of inner conductor 250, elongate conductive path 238 and probing needle 164. Preferably, the test equipment is connected so that a feedback circuit in the output channel of the test equipment supplies a “guard” voltage that matches the instantaneous signal voltage, which guard voltage is applied to outer conductor 256 and to conductive land 244. By use of the cable 180, additional shielding is achieved by nearly eliminating the capacitance associated with the prior art use or long traces on the probe card to route the signal from the pad sets 170 to the probing devices 154. Accordingly, the cable 180 reduces the leakage currents and settling time.
Moreover, with respect to probing devices 154, each elongate conductive path 238 is guarded by conductive plate 240 on the opposite side of the blade 236 and by the corresponding conductive land 184 which is arranged below the path. By minimizing leakage currents into and out of each elongate path 238, this guarding system reduces the levels of undesired background current and so enhances the effect achieved by the use of cables in suppressing leakage currents and reducing settling times.
In one variant, feature 386 is the “chuck” or “return” guard. This chuck or return guard is described in greater detail in U.S. Pat. No. 5,345,170, which is assigned to the assignee of the present application and is incorporated by reference into the present application as if fully set forth herein. In the case where the return signal path is through the wafer and into the chuck, this allows for a return guard path from the chuck guard that is the same as the guard of the probe card.
In a second variant, feature 386 is the same as shield 182, so that layer 376 forms a bottom shield for card 152.
In a third variant feature 386 is an instrument channel set to drive layer 380 to parallel the potential of whichever signal channel was actively engaged in forming a measurement. In this application layer 376 or 384 would be connected to a test instrument channel that would drive layer 376 or 384 to the voltage of whatever trace set was actively engaged in a measurement. This would reduce noise and settling time. The second and third variants perform the important additional function of shielding or guarding, respectively, the test subject workpiece. This task is rendered comparatively more difficult in the pogo probe environment because of the 21 cm (8.5 in) aperture required to accommodate the introduction of the pogo probe head into the shielded box that is used in ultra-low current testing. By placing a guard or shield or both in the bottom portion of the probe card 152, the test subject workpiece may be shielded or guarded or both despite the presence of this aperture.
In another set of three variants, feature 388 would represent either the guard chuck, shield 182, or the test instrument channel set to mimic the active measurement channel. Typically, conductive layer 376 and 384 (either one of which may be omitted) would be continuous over the area of board 158, except they would be cut away to avoid contact with vias 206. In the third variant the additional layer 376 or 384 may be divided into additional auxiliary guard traces.
Dielectric layers 200, 378, and 380 are all about 1 mm (39 mils) thick. Dielectric layer 202 (
Yet another embodiment is shown in
Each pair of auxiliary guard traces 434 is separated by a set of trenches 436. Similar to trenches 210 trenches 436 separate auxiliary traces 434 and reduces cross capacitance between pad sets 420 and traces 428 and 432 thereby reducing cross-talk and settling time. This embodiment may be somewhat less expensive to produce in large numbers than the previously described embodiments.
Skilled persons will further readily recognize that the connection structure of connectors 506 and 508 could be cut away and that the center connectors of 250 a and 250 b of cables 180 a and 180 b (
With respect to the detailed structure of holder 502, an elongate probe connector 552 is conductive and preferably has a rectangular cross section. An insert 554 is sized to fit within the probe connector 552. Insert 554 includes a ceramic insulator 556 and a conductive bent connector 558 attached to one side of the insulator 556. Insulator 556 is in face-to-face abutment with the interior upright surface 559 of probe connector 552. Probing device 504 is matingly and detachably engageable within the probe connector 552.
Referring also to
In the particular embodiment shown, the blade 504 is generally L-shaped in profile and is edge-mounted within the probe connector 552 so that the short arm of the L-shaped blade 504 extends downwardly making contact with the test subject device.
Referring also to
The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.